Spark producing apparatus including saturable core transformer



J. V. M NULTY July 21, 1959 SPARK PRODUCING APPARATUS INCLUDING SATURABLE CORE TRANSFORMER Filed NOV. 23. 1953 2 She ets-Sheet 1 INVENTOR. JOH/V 1 fl/c/Vum ATTO/Q/VEV July 21, 1959 J. V. MCNULTY SPARK PRODUCING APPARATUS INCLUDING SATURABLE CORE TRANSFORMER Filed Nov. 23. 1953 2 Sheets-Sheet 2 LEJ INVENTOR.

United States Patent SPARK PRODUCING APPARATUS INCLUDING SATURABLE CORE TRANSFORMER John V. McNulty, Norwich, N.Y., assignor to General Laboratory Associates, Inc., Norwich, N.Y., a corporation of New York Application November 23, 1953, Serial No. 393,831 1 Claim. (Cl. 315--177) This invention relates to spark producing apparatus, and especially to apparatus of the type used to ignite fuel and air mixtures. The particular apparatus described herein is intended for use on jet engines. Many of the features of the invention are of particular utility in connection with such engines, while other features are useful in spark producing apparatus generally.

Jet engines present difiicult ignition problems. The pressure in the combustion chamber is relatively low, being commonly of the order of twenty pounds per square inch gage pressure. The temperature is also low, being only a few degrees above the ambient temperature, which, especially in the case of aircraft operating at high altitudes, may be very low. There is commonly great tur bulence in the neighborhood of the spark plug. There is also a wide variation in the fuel-air ratio of the mixture presented for ignition.

Because of these problems, it has been recognized that to secure reliable ignition in such engines, the ignition apparatus must create at the spark plug sparks of sub stantial volume and of high energy content. Such sparks must have suificient volume that they will affect enough fuel to initiate the propagation of a flame, even though the fuel-air ratio may be substantially lower than the optimum required for such purposes.

In order to get a large volume of spark, it is necessary to have a long spark gap. It is well-known that the breakdown potential of a gap increases with the gap length. Jet engine ignition systems therefore require long spark gaps and high breakdown potentials. At the same time, the necessity for greater energy in the spark requires large current flows in the ignitionimpulses.

These two requirements present conflicting problems in the construction of such ignition systems. The require ment for high potential leads to the use .of high impedance circuit elements. However, such high impedance elements cut down the flow of current and tend to reduc the peak spark energy.

Jet engine ignition systems of the prior art commonly operate at a frequency in the negihborhood .of one or two megacycles. The inductive elements in these prior art systems must have comparativelylarge numbers of turns. Furthermore, at such high frequencies, the dis tributed impedance of the leads between the circuit com ponents is a substantial proportion of the total load on the system. Consequently, there may be a substantial reduction in performance if the loading effect of the distributed impedance is too great. This reduction in performance may be characterized by a reduction of potential at the spark gap or by a loss of energy.

A currently popular type of ignition system includes a plying a breakdown impulse to the spark gap and a lower i main energy discharge to the gap after it has been broken high frequency circuit including a transformer for supdown by the high frequency circuit. Such systems take advantage of the fact that the breakdown voltage required to initiate a spark discharge across a gap is considerably higher than the sustaining voltage required to keep the discharge going.

In ignition systems of the type just described, it is common to store the electricity for the main spark discharge on a condenser and to connect that condenser to the spark plug, by a path which is continuously con ductive. Because of the impedance of the spark plug gap, the energy stored on the condenser does not leak off until the spark gap is broken down by the high frequency, high voltage breakdown impulse. In such systems, difliculty is sometimes encountered when the plug is fouled. The fouling material deposited on the plug will provide a leakage path in such a system, through which a substantial part of the charge stored on the condenser may leak off before the time when a spark is required.

There are two general types of spark plugs now in use in jet engines. One type is the common air gap plug, which requires a high voltage to break it down. The other type is the less common surface gap plug. A surface gap plug includes two electrodes separated by a surface having a coating of semi-conductive material. When a sufiiciently high current flows through this mate.- rial, it is heated to a point where it emits electrons in the gap between the electrodes. The emission of these electrons ionizes the air between the electrodes and substantially reduces the breakdown potential between the electrodes. A surface gap plug requires a substantial initial current in order to break down the gap and form a spark as contrasted to the high initial voltage requirement of the gap type of plug.

An object of the present invention is to provide an ignition system which will operate at a substantially lower plug breakdown voltage and at a lower frequency than systems of the prior art.

Another object is to provide an ignition system in which the loading eifect of the leads is an insignificant part of the total load on the circuit, so that leads of varying lengths may be used. Another object is to provide a system which may be used with either air gap or surface gap plugs.

Another object is to provide an ignition system including a closed core transformer having a small number of turns and a high impedance.

Another object of the invention is to provide an improved ignition system of the type in which a high voltage breakdown impulse is transmitted to a spark plug and followed by a high energy impulse from a condenser, in which improved system the current flow path between the condenser and the spark plug is not continuous, but is intermittently closed.

A further object is to provide a system of the type described in which the current path between the condenser and the spark plug is periodically completed by a switching device. A further object is to provide a system of this type in which the current path is periodically closed by the breaking down of a sealed spark gap. A further object is to provide a system of this type in which the closure of the conductive path is controlled by a relay responsive to the condition of the charge on the condenser.

The foregoing and other objects of the invention are attained by providing a novel ignition system described below. This system includes a condenser, means for charging the condenser and means for periodically connecting the condenser to a discharge circuit. This discharge circuit includes a transformer having a'saturable core. During the initial phase of the discharge of the condenser, an oscillating current flows through the primary winding of the transformer, thereby inducing a high voltage in the secondary winding, which is connected in series with the plug. This high voltage breaks down the gap at the plug. As soon as the gap breaks down, current flows directly from the condenser through the secondary winding. This current flow quickly saturates the core of the transformer so that its impedance becomes negligible and a heavy discharge of current from the condenser through the gap follows.

. The foregoing and other objects of the invention will become apparent from a consideration of the following specification and claim and the accompanying drawings.

In the drawings:

Fig. 1 is a wiring diagram of an ignition system embodying the invention;

. Fig. 2 is a fragmentary wiring diagram illustrating a modification of the system of Fig. 1;

Fig. 3 is a plan view of a transformer employed in the system of Fig. 1;

- Fig. 4 is a view partly in elevation and partly in section on the line IVIV of Fig. 3, and

Figs. 5, 6 and 7 are wiring diagrams of ignition systems embodying modifications of the invention.

FIGURE 1 There is shown in the drawing a battery 1 supplying electrical energy to an exciter unit 2 connected to a compositor unit 3, which in turn delivers a sparking impulse to an ignition plug 4.

The exciter 2 includes a filter network 5, a motor 6 driving a multiple lobe cam 7 and a single lobe cam 8. The exciter 2 also includes autotransformer 10, a rectifier 11 and a resistor 12. The exciter unit also includes three condensers 13, 14 and 15.

The compositor unit 3 comprises a saturable core transformer 16 including a core 17, a primary winding 18 and a secondary winding 19. The transformer 16 is preferably constructed as shown in Figs; 3 and 4. The core 17 is first covered with a sheath 20 of insulating material and the secondary winding 19 is wound directly on that sheath. The core with the coil 19 Wound on it is then placed in a mold and covered with a molded insulating cover 21. The primary winding 18 consists simply of three turns of flat strip copper wound around the outside of the molded cover 21. I

The ignition filter network is of conventional construction, including three inductive reactors 22, 23 and 24 in series and condensers 25, 26, 27 and 28 connected between the reactor terminals and the ground.

Operation-Fig. 1

The battery 1 supplies energy to the motor 6 through an obvious circuit which includes the filter 5, a conductor 29, motor 6, and ground connections 30 and 31. The motor 6 runs continuously when the system is in operation. The multiple lobe earn 7 repeatedly opens and closes a switch contact 32. The condenser 14 is connected across the switch 32 and serves to quench or eliminate the sparks which might otherwise occur at that contact.

The battery 1 also supplies electrical energy to a primary ignition circuit which may be traced through the filter network 5, conductor 29, a conductor 33, switch 32, the primary section 34 of the winding of autotransformer 10, and ground connections 35 and 31 to the opposite terminal of battery 1. Current flowing periodically in this primary circuit induces an electrical potential in the secondary section 36 of the winding of autotransformer 10. Current flows from that secondary section 36 through a secondary ignition circuit including rectifier 11, condenser 13 and ground connections 37 and 35. As the contact 32 is repeatedly opened and closed, electrical impulses of proper polarity to pass the rectifier 11 are stored on the condenser 13, hereinafter sometimes referred to as the storage condenser. This apparatus for charging the condenser 13 is an improved form of the apparatus completely disclosed and claimed in my copending application for U.S. patent Serial No. 227,721, filed May 22, 1951, entitled Engine Ignition Apparatus and Procedures, now Patent No. 2,716,720, issued August 30, 1955.

After the motor 6 has completed one revolution of the cams 7 and 8, the cam 8 closes a trigger switch contact 38, thereby completing a trigger circuit which may be traced from the condenser 13 through contact 38, wires 39 and 40, primary winding 18, wire 41, trigger condenser 15 and ground connections 42 and 37 back to the condenser 13. This circuit produces an oscillating discharge of the condenser 13. This oscillating current flowing in primary winding 18 induces a high potential in secondary winding 19, which is impressed across the 'gap of the ignition plug 4, this potential being suflicient to break down that gap and initiate a discharge of electricity across it. The circuit through which this potential is impressed may be traced from condenser 13, through contact 38, conductors 39 and 40, secondary winding 19, wire 43, the gap of spark plug 4 and ground connections 44 and 37. The impedance of this circuit, hereinafter referred to as the main discharge circuit, is reduced by the breakingdown of the gap at the plug 4, and current begins to flow directly from the condenser 13 through the same circuit. This current sets up a flux in the core 17 of the transformer 16, which is sufiicient to saturate that core. The impedance of winding 19 is thereby quickly reduced to a very low value and the condenser 13 is substantially completely discharged very quickly through this low impedance circuit to the spark plug 4. The complete action of the primary and secondary circuits associated with transformer 16 takes place while the contact 38 is closed. The contact 38 remains closed long enough for condenser 13 to discharge substantially completely. As soon as contact 38 opens, both circuits are interrupted and the battery 1 starts charging the condenser 13 again through the cooperation of contact 32, transformer 10 and rectifier 11.

The resistance 12 is connected between the junction of wires 39 and 40 and a ground connection 45. This resistance is provided to discharge the condenser 15 during the time that the contact 38 is open, so that the condenser 15 will be completely discharged when the contact 38 closes and cannot block the flow of current from condenser 13. Condenser 15 might acquire such a blocking charge, for example, if there were an accidental open circuit at the ignition plug 4.

It may be seen from the foregoing that there is no continuous conductive path between the condenser 13 and the plug 4. This path is normally open at the contact 38, and is closed only when a spark is desired at the plug 4.- i The time of closure of the contact 38 is made long enough for the condenser 13 to discharge substantially completely through the low impedance circuit including the secondary winding 19 on the saturated core 17. The time of closure is so designed in order that the condenser 13 may be substantially completely discharged before the contact opens and the contact 38 is therefore not required to interrupt any substantial current flow.

The trigger condenser 15 acts as a blocking condenser both before and after saturation ofthe core 17. Before saturation of the core, condenser 15 prevents the loss or leakage of charge from condenser 13 through the primary winding 18. Any charge passing through the winding 18 is then stored on the condenser 15. After the core 17 is saturated, condenser 15 is charged substantially to the same terminal voltage as condenser 13. It then adds its charge to that of condenser 13 and helps to supply the main discharge through the plug 4. When the core 17 becomes saturated, the primary winding 18 loses substantially all its inductance, which thereby blocks the oscillation in the primary circuit which started during the initial or pre-breakdown phase of operation.

The following table sets forth the characteristics of the principal circuit elements in one particular ignition system constructed in accordance with the present invention:

Condenser 13 2.0 mfd.

Condenser 14 r 1.0 mfd.

Condenser 15 .25 mfd.

Resistance 12 2000 ohm resistor, Watt rating.

The transformer 16 in this system consisted of an annular core of powdered and sintered magnetic material, with a core cross-section having dimensions of .445 by .550 inch. The secondary Winding consisted of 78 turns of #22 wire, and the primary winding consisted of 3 turns of a flat copper strip equivalent to a #18 wire.

The transformer structure shown, with an annular core, gives a high mutual inductance between the windings. The trigger circuit described above is a high impedance circuit, since the core is not saturated when the trigger circuit is effective. Furthermore, the discharge circuit is of low impedance, since the core is saturated when that circuit is effective. These characteristics permit the use of a high turns ratio with a low number of turns. The trigger circuit is a high Q circuit. In the particular system described above, the ratio of reactance to resistance at the operating frequency is such that the voltage across the coil 18 is substantially double the voltage impressed on the circuit by the condenser 13. This doubling of the voltage and the high turns ratio of the transformer result in a substantial increase in the voltage across the spark gap.

The trigger circuit of the present invention has a lower frequency than is common in the ignition systems of the prior art, and still the impedance of the system to the high energy discharge is maintained at a low value. In the particular system just disclosed, the trigger frequency is approximately 100 kilocycles as compared to 1 or 2 megacycles in conventional ignition systems. This has a substantial effect in lowering of the breakdown potential at the spark gap. The breakdown potential of any gap varies with the time during which the potential is impressed across it. For example, a particular value of unidirectional potential continuously maintained will break down a gap of given dimensions. An impulse of alternating potential must have a greater peak value than this continuously maintained unidirectional potential in order to break down the same gap. The particular peak value of alternating potential required will depend upon the wave form, i.e., the length of time during which the peak potential is maintained on the gap. This relationship is commonly expressed in terms of a factor termed impulse ratio or impulse error, namely the ratio between the breakdown voltage for any particular wave form and the unidirectional breakdown voltage. Generally speaking, the lower the frequency, the lower this ratio becomes,

for any given wave form. Consequently, it may be seen that the lower frequency circuit described can break down a gap at a potential lower than that required by a higher frequency trigger circuit.

Because of the high impedance trigger circuit, the lead impedance between condenser 13 and primary Wind- 6 ing 18, is not an appreciable factor in the operation of the system described. This lead impedance is so much lower than the transformer primary impedance that it is-negligible as a factor in the operation of the system.

The output leads in the system just described may be made longer than in conventional systems without adverse eifects, because of the lower frequency employed and because of the consequent lower attenuation of ignition impulses.

FIGURE 2 This figure illustrates a modified form of ignition system embodying the invention. This system includes two spark plugs in series which are energized from a single condenser. It also illustrates a further modification of the circuit of Fig. 1 in that a sealed gap is used to control the timing of the ignition impulses instead of the motor driven switch of Fig. 1.

Referring to Fig. 5, there is shown a battery supplying current to a transformer 51 having a primary winding 52 and a secondary winding 53. A vibrator type interrupter 54 is connected in series with the battery 50 and primary winding 52, in order to provide a flow of alternating current in that winding. Other common types of connections for the interrupter may be used. The secondary winding 53 is connected through a rectifier 55 to a storage condenser 56.

Condenser 56 serves as a cource of electrical energy for two spark plugs 57 and 58. Two saturable core transformers 59 and 60 are provided for controlling the discharge through the plugs 57 and 58. The discharge circuit for condenser 56 includes a sealed gap 61 and .primary and secondary circuit branches which are connected in parallel to the condenser 56 and the gap 61. The primary circuit branch may be traced from gap 61 through primary windings 62 and 63 of the transformers upper terminal of condenser 56, through gap 61, secondary winding 67 of transformer 59, the two spark plugs 57 and 58 in series, secondary winding 63 of transformer 60 and thence to the opposite terminal of condenser 56. The last-mentioned terminal is isolated from ground by condenser 69. The common terminal of the twospark plugs 57 and 58 is also grounded at 70. It will be recognized that the two spark plugs may be located some distance apart with separate ground connections. The grounding of the common terminal of the spark gaps permits the use of shielded cable to connect both plugs to the ignition system, thereby reducing generation of radio interference.

Operation of Fig. 5

The operation of this system is analogous to that of the system of Fig. 1. When the system is placed in operation, the condenser 56 starts to charge. As soon as the voltage on the condenser 56 exceeds the breakdown potential of gap .61, a discharge takes place through that gap and through the primary branch described above, thereby producing in the secondary circuit branch high breakdown potentials which initiate discharges at the spark plugs 57 and 58. When these discharges are initiated,

current flow then takes place through the secondary cira discharge through the gap 61 and the plugs 57 and 58 in series, whereupon the discharge ceases and the con- -denser 56 starts to charge again.

The balance choke coil 64 is provided to prevent a short circuit at one of the two spark plugs reflecting on and effectively shorting out the other plug. For example,

:if the plug 57 is shorted by fouling deposits. so that it draws a heavy current through the secondary Winding 67, the resulting current flow in the primary winding 62 will appear also in the adjacent section of the balance choke coil 64. The balance choke coil then acts in a manner analogous to an autotransformer, inducing a potential in its oppsite section which effectively increases the potential across the primary winding 63. The sudden drop in potential across winding 62 is therefore ineffective to reduce the potential across winding 63. Consequently, the plug 58 will still receive its normal high frequency breakdown impulse, even though the plug 57 is substantially short-circuited.

FIGURE 6 This figure illustrates a modified form of the circuit of Fig. 1, including particularly a modified arrangement for timing the initiation and termination of the ignition impulses.

alternator 71 through a conventional filter network 72, a

transformer 73 and a rectifier bridge 74.

Connected across the output terminals of the rectifier bridge 74 is a storage condenser 75 corresponding in function to the storage condenser 13 of Fig. 1 and the storage condenser 56 of Fig. 5. The charging circuit for condenser 75 may be traced from one output terminal of the bridge 74 through condenser 75, the winding of a relay 76 and a condenser 77 in parallel with that winding, to the opposite output terminal of the rectifier bridge 74.

The discharge circuit of condenser 75 is similar to the discharge circuit of condenser 13 of Fig. 1. Those elements which are common to both circuits have been given the same reference characters and will not be further described. The only difference between the two circuits is that the contact 38 of Fig. 1 is replaced in Fig. 6 by a contact 78 operated by the relay winding 76.

When the condenser 75 is charging, current flows through the relay winding 76 and holds the contact 78 open. When the charge of condenser 75 reaches a point such that its terminal voltage approximates that of the rectifier bridge circuit 74, then the flow of current through relay winding 76 drops to a point where it can no longer hold the contact 78 open and the contact 78 then closes, completing the discharge circuit for condenser 75 through the ground connection 79.

The discharge of condenser 75 proceeds in a manner similar to that of discharge of condenser 13 of Fig. 1. After the condenser is substantially completely discharged, it starts charging again, and the flow of charging current through relay winding 76 causes it to pick up contact 78, thereby opening the discharge circuit again. The relay winding 76 has a characteristic time delay between its energization and the lifting of its contact 78 suthcient to permit the discharge of condenser 75 to proceed substantially to completion before the contact 78 is opened.

FIGURE 7 'a storage condenser 80 which is supplied with electrical energy from a transformer secondary winding 81 through a rectifier 82. The secondary winding 81 may be the when the spark gap is secondary portion of an autotransforrrier winding such mary circuit branch comprising in series a sealed air gap 83, the primary winding 84 of a saturable core trans- "former 85, and acondenser 86 connected in parallel with "a resistor 87. The discharge circuit of condenser also "includes a secondary branch including a sealed air gap 88, a secondary winding 89 of the saturable core transformer 85, and a spark gap 90.

The sealed air gap 83 determines the potential at which condenser 80 starts to discharge. The operation thereafter is similar to that of the circuit of Fig. 1, for example, except that the discharge through the air gap 83 terminates when the difference between the potentials across condensers 80 and 86 becomees less than the discharge sustaining voltage. of gap 83. This occurs after the core of transformer 85 has become saturated, and prevents loss of energy in the primary circuit branch 84.

By calibrating the air gap 83 carefully, it may be made to regulate the timing of the sparks at the spark gap and also to minimize the loss of energy in the primary circuit branch. The only energy dissipated in the primary circuit branch is that necessary to initiate the breakdown of the spark gap 90. As soon as the spark gap 90 breaks down the core 85 saturates, the potential on condenser 86 builds up until the sealed gap 83 stops conducting and no further energy is dissipated in the primary .circuit branch.

When the spark gap 90 is one at which fouling conditions may be expected, another sealed spark gap 88 is connected in series with the secondary circuit branch. The gap 88 should preferably have a greater breakdown voltage than the gap 83. It is effective under conditions of fouling at the gap 90 to ensure that a high voltage discharge takes place at the gap 90 in spite of the fouling conditions. If the gap 88 were omitted, the condenser 80 might be effectively shorted by the fouled gap 90.

If no fouling conditions are anticipated at the gap 90, the sealed gap 88 may be eliminated.

While I have shown and described certain preferred embodiments of my invention, other modifications thereof will readily occur to those skilled in the art, and I therefore intend my invention to be limited only by the appended claim.

I claim:

Spark producing apparatus, comprising a transformer having primary winding means, secondary winding means, and a saturable magnetic core; a spark gap having a fixed ;breakdown potential, a trigger condenser, primary and .secondary circuit branches, said primary circuit branch comprising said primary winding means and said trigger condenser in series, said secondary circuit branch comprising said secondary winding means and said spark gap in series, an energy supply condenser, means for charging said energy supply condenser, means operable to connect said energy supply condenser across said primary circuit branch, said last-named means including a first sealed gap connected in series with the primary winding means and the trigger condenser and having a predetermined breakdown potential, saidenergy supply condenser being effective upon breakdown of said first sealed gap to provide a current flow through said primary winding, said energy supply condenser and said transformer then cooperating to induce in said secondary winding a potential efiective to break down said spark gap, means operable to connect said energy supply condenser across said secondary circuit branch, said last-named means including a second sealed gap having a breakdown voltage greater than that of the first sealed gap and less than that of the spark gap, said energy supply condenser being effective broken down to produce a current flow through said secondary winding sufliciently large to 10 saturate the core and thereby to reduce the impedance of References Cited in the file of this patent the secondary winding and to increase cumulatively the UNITED STATES PATENTS current through the spark gap untll the energy supply 1,501,485 Hunt y 1924 condenser potential falls to a value too low to maintain a spark discharge at the gap, said first sealed gap having 5 1,557,201 Hunt 13, 1925 an arc sustaining potential greater than the potential exist- 1,598,486 Mallory 31, 1926 ing across the energy supply condenser when the core is 2,392,192 Robinson Jail- 1, 1946 saturated, said first sealed gap being thereby effective to 2,589,164 Tognola Mal 1952 open the primary circuit branch before the core becomes 2,651,005 TPgnola P 1, 1953 saturated. 10 2,737,612 Sims Mar. 6, 1956 

